Obstructive sleep apnea (OSA), a term for periods during sleep when breathing is blocked or impeded, is a highly prevalent medical condition, increases with obesity and diabetes, and affects 9-18% of the adult U.S. population. OSA occurs because of recurrent upper airway collapse during sleep leading to reductions in airflow with cyclic changes in body oxygen, and results in a state of chronic intermittent hypoxia. Many essential processes, such as production of red blood cells, energy regulation, and formation of new blood vessels, are regulated by hypoxia. OSA is an exceedingly common problem that has been linked to high blood pressure, diabetes, poor memory and heart attacks. Hypoxia leads to up-regulation of hypoxia-inducible factors (HIFs). HIF-1 and HIF-2 were discovered as a result of studies of erythropoietin (EPO), the key hormone that stimulates erythroid progenitors and regulates the production of erythrocytes, the subject of the PI's previous VA support. This application is designed to gain novel insights into the contribution of HIFs, which regulate erythropoiesis, to the pathophysiology of OSA. The PI of this proposal has been intrigued that, in contrast to other hypoxic conditions, his major clinical/academic focus of polycythemia/erythrocytosis is not a common feature of OSA. During prolonged hypoxia, HIFs mediate an increase in erythropoiesis, leading to an increased red blood cell (RBC) mass. Upon return to normoxia, the increased RBC mass is abruptly overcorrected by the preferential destruction, i.e., hemolysis, of hypoxia-formed young RBCs, a phenomenon termed neocytolysis. We created a novel mouse model of neocytolysis and used this model to show that neocytolysis is mediated by excessive accumulation of reactive oxygen species (ROS), increased mitochondria in RBCs, and increased micro RNA miR-21 which down-regulates catalase. We also obtained preliminary data suggesting that neocytolysis is the main mechanism preventing polycythemia in patients with OSA. We observed that erythrocytes and reticulocytes of OSA patients have reduced catalase transcripts and activity, along with increased miR-21, and that these levels normalize with CPAP treatment. We also observed that in uncorrected OSA, increased mitochondrial mass and levels of ROS are found not only in reticulocytes and mature RBCs, but also in other blood cells such as platelets, T-cells, B- cells, granulocytes, and mononuclear cells. With CPAP treatment, mitochondrial ROS decrease and mitochondrial mass normalizes. To accomplish our research goals outlined in this research proposal, we will elucidate OSA changes in HIF-regulated pathways using peripheral blood cells to confirm our preliminary findings that neocytolysis occurs secondary to chronic intermittent hypoxia in most OSA patients. We will determine the metabolic consequences of chronic intermittent versus sustained hypoxia in a mouse model of neocytolysis and assess the effect(s) of pharmacological manipulation of HIFs. We will evaluate the effects of carbon monoxide (CO), which is increased by hemolysis, on the activity of the carotid body (CB). The CB is the key sensor of arterial blood oxygen. Hypoxia increases neural signals from the CB which, through chemosensory reflexes, controls breathing and blood pressure, a principal adaptation to hypoxia. Enhanced CB chemosensory reflexes play a substantial role in OSA pathophysiology. Emerging evidence suggests that hypoxic sensing by the CB involves blood oxygen-dependent interplay between CO generation by heme oxygenase-2 and hydrogen sulfide synthesis by cystathionine-?-lyase. We will evaluate, using an exhaled CO end-tidal breath analyzer, whether increased CO produced by hemolyzed RBC in OSA influences CB sensing and ensuing chemosensory reflexes.